Light weight, tear resistant, waterproof fabrics are desired for many end uses including but not limited to: tents, tarps, awnings and marine covers. In order to achieve a moisture barrier, fabrics have traditionally been very tightly woven or coated with a waterproofing resin. In either case, the fabric structure and individual yarns are immobilized and the tear strength is reduced, necessitating using a heavier than desired fabric for a particular use. Coated and tightly woven fabrics not only are heavy and difficult to use and store, but are also subject to tear, particularly along creases where the fabric has been folded.
In addition to waterproofing other fabric properties are of interest and can be achieved by proper selection of the fibers from which the fabric is woven and the use of additional treatments. For example, it has been desirable to make fabrics used for certain tents, tarps, awnings and the like out of blends which are predominantly cotton since cotton fabric will char and not melt when exposed to heat. Flame retardant additives are frequently added as treatments to these fabrics increasing their safety.
For fabrics used in applications such as tents, tarps, awnings and marine covers resistance to tearing is an important property. In previous attempts to increase the tear strength of such fabrics, yarns from high strength reinforcing fibers such as p-aramids, particularly fibers of poly(p-phenylene terephthalamide) PPD-T, have been incorporated into the fabric structure to improve the tear strength as described, for example, in Japanese Publication Kokoku Sho 62-26900 which discloses the use of from 1-40% of an aromatic polyamide yarn that is preferably at least 70% p-aramid and has a tensile strength of at least 15 gpd or alternatively, more than twice the tensile strength of the yarns of the fabric to be reinforced. Even with the use of such high strength yarns as those of PPD-T, Japanese Publication Kokoku Sho 62-26900 teaches that 2-5 adjacent ends of yarn are required to achieve adequate tear resistance.
An alternative method of improving the strength of a fiber has been to make intimate blends of the p-aramid staple with a natural staple fiber such as cotton or synthetic staple fibers such as nylon or polyester as disclosed in, for example U.S. Pat. No. 4,941,884 or U.S. Pat. No. 4,900,613. These references postulate that the best increase in tear strength is to be obtained by using a reinforcing yarn of maximum strength having low break elongation such as the p-aramid yarns.
Another alternative method of reinforcing the tear resistance of fabrics is disclosed in GB 2,025,789 where the tensile strength and the tear resistance of a coated fabric is improved by incorporating ends, either as single yarns or as groups of yarns, having higher strength and lower elongation than that of the background yarn in the weave of the coated fabric. GB 2,025,789 asserts that the lower elongation of the reinforcing yarn is a requirement for achieving a tear resistance improvement by enabling the stronger reinforcing yarns to take up the stress before the stress acts on the yarns of the background fabric.